The present invention relates generally to web processing apparatus and, more particularly, to an apparatus for determining the approximate web tension setting necessary for providing a predetermined web width in a web processing machine, such as a web printer, which is used to process a continuous plastic web of the type which is subject to significant stretching and shrinking in response to relatively small web tension variations.
Composite webs such as described in U.S. Pat. No. 4,254,173 for COMPOSITE MATERIAL FOR SECONDARY CONTAINER PACKAGING of A. Dean Peer, Jr. (which is hereby specifically incorporated by references for all that it discloses), consist of a web of paper material adhered to a web of plastic film material. In the composite web forming process described in the Peer patent, the film web, prior to being laminated to the paper web, is printed with repeating graphics material which provide the visual display for packaging cartons or the like which are formed from the web in subsequent processing steps. In the manufacture of composite webs of the type described in Peer, a considerable cost savings may be achieved by use of "non-oriented" plastic films as opposed to "oriented" plastic films. However, non-oriented plastic films are considerably less stable than oriented films, i.e. non-oriented plastic films are much more susceptible to stretching both in a length and width direction in response to minor tension variations, temperature changes, etc., than oriented plastic films.
Methods and apparatus for measuring and controlling the dimensions of a web of plastic film prior to its lamination to a web of paper are disclosed in U.S. Pat. No. 4,496,417 of Haake et al.; U.S. Pat. No. 4,572,752 of Jensen et al.; U.S. Pat. No. 4,610,739 of Jensen, and U.S. Pat. No. 4,786,353 of Templeton et al., which are each hereby specifically incorporated by reference for all that is disclosed therein. The above referenced patents describe laminating devices which measure the relative elongation and/or the width of a repeat length portion of a moving web of relatively stretchable plastic film material The measurements thus made are used by an associated control system to vary an operating parameter of the system to maintain the web repeat length or width near a predetermined design value. However, it has been found that such control systems are most effective if the initial web tension setting of the associated web processing apparatus is approximately correct for the particular plastic web which is being processed. If the system tension setting is significantly different than the "ideal" tension setting for the particular web being processed, then the control system may experience trouble with accurately controlling the subject web parameter, i.e. web width or repeat length.
A similar problem exists in the printing of plastic film web which is to be laminated to a paper web by a lamination process such as described in U.S. Pat. No. 4,496,417 of Haake et al. Plastic film webs, when subjected to high tension forces in the longitudinal direction, tend to stretch in a longitudinal or lengthwise direction and, at the same time, tend to "neck down," i.e. decrease in width. It is desirable to print webs at a tension setting which provides a web width approximately equal to the desired web width of the finished laminated product. However, in the printing of non-oriented plastic film webs, the printing machine tension setting which is needed to provide a desired web width varies from roll to roll of web material. If the initial tension setting is approximately correct, then the control system provided on the printing machine can thereafter monitor web width and make small adjustments to web tension to provide the desired width. However, if the initial web tension setting is substantially different from the tension setting required to produce a desired width, then such printer control systems are often incapable of accurately providing the tension correction which is required.
Due to the fact that the behavior of non-oriented plastic films varies significantly from roll to roll, even when films of the same specification are purchased from the same manufacturer, it is quite important to accurately set the initial processing machine operating tension for each new roll of plastic film which is to be processed. Prior to the present invention, the method for determining an appropriate web tension setting for a particular roll of plastic film was to submit a sample of the film to a standardized elastic modules test such as described in attached Appendix A, which is hereby specifically incorporated by reference herein and which forms a part of the disclosure of this specification as though fully set forth herein. A problem with the use of such a test is that it is relatively expensive to perform, e.g. a typical lab charge for performing this test on a single roll of plastic film during 1988 is $150.00. Another problem is that such a test necessitates access to a laboratory with sophisticated testing equipment and highly skilled technicians. Another problem is that, even when such a laboratory is nearby, the testing often takes a considerable period of time to perform, e.g. ten days between the submission of samples and the return of the test results. Thus, a web processor is required to maintain a considerable inventory of plastic web rolls due to the lag time between the submission of test samples and the return of results.
Thus, a need exists for a method of quickly, accurately, and inexpensively testing a roll of plastic film material for determining a proper initial tension setting for a web processing machine such as a laminator or printer.